Hi Frank,
1. It is not possible to write seriously on the navaja without drawing heavily on the work of Forton. As far as I am aware, there is only one other work worth considering, that of Arturo Sanchez De Vivar, titled La Navaja Clasica. It is a simple and concise work that also drew heavily on Forton and it was written to address mainly the needs of collectors. At least in Spanish, I consider it overshadowed by Forton's Las Mejores Piezas de Colecsion. May be worth translating, but with the same effort a more comprehensive book could be written. However even such a modest work would be a financial risk in terms of publishing costs, especially that of photography which would require going to Spain and hiring a local photographer.
2. The Spaniards claim that they invented the navaja and others copied its design. This may or may not be true, but remarkably similar clasp knives were in use in other parts of Europe, mostly in France and Italy and generally of significantly better quality.
Here is a superb Italian navaja:
http://www.knivescollection.com/cata...d_coltello=631
It was probably an early 19th century exhibition piece, with a 44cm (17") blade that was provided with a sheath, presumably so that it could be carried in the open position if the owner so desired and if the jurisdiction's law permitted it though the long handle would have made this very uncomfortable.
I strongly suspect that the Spaniards were not the only ones to have legislated against fixed blade knives, for why else would the Italians and the French, just to mention two , bothered with inferior (to a fixed blade) folders? That these nations also passed restrictive legislation of some sorts is evidenced by the old and curious practice of selling navajas with the blade's tip sporting a blunt appendage which the owner ground off to end up with the desired sharp point; This was done to circumvent the prohibition of selling pointed clasp knives. The attached phot is that of such an Italian navaja that never saw use and was sold some time ago by the internet firm (knivescollection) given above.
It is my opinion that to make a truly comprehensive study of the navaja one would have to broaden the scope of inquiry so as to include other European nations, besides Spain; However, linguistically this would require an unusually gifted researcher who is capable of looking through old police records and similar archival documents written in a number of different languages. This is why we cannot find anything worth reading in English on European knives and their usage.
As you rightly mention, there were navajas made even as afar as India, though that was around 1900. The basic design lending itself to being manufactured in low tech workshops.
Cheers
Chris